1. Field of the Invention
The present invention relates to a developing device for a copier, facsimile apparatus, printer or similar image forming apparatus and, more particularly, to a developing device of the type developing an electrostatic latent image formed on an image carrier with a thin layer of one component type developer formed on a developer carrier.
2. Discussion of the Background
A conventional developing device of the type described is located to face a photoconductive belt or similar image carrier. The device has a developing roller or similar developer carrier. A conductive blade contacts or adjoins the surface of the developing roller at the edge thereof and plays the role of a developer regulating member. A discharge brush is located downstream, with respect to the rotation direction of the roller, of a developing region where the belt and roller contact each other. The discharge brush contacts the roller at such a position and serves as surface potential stabilizing means. A power source is connected to the roller, blade, and discharge brush. Developer supplying means supplies a developer to the roller. The roller carries a developer fed from the supplying means thereon and conveys it toward the belt. At this instant, the blade regulates the thickness of the developer deposited on the roller, thereby forming a thin developer layer. When the developer passes through the gap between the blade and the roller, it is charged to a predetermined polarity due to the friction thereof with the surface of the roller and the edge of the blade. As the charged developer arrives at the developing region due to the rotation of the roller, it is transferred to the belt to develop a latent image electrostatically formed thereon. Subsequently, the charge remaining on the roller is dissipated by the discharge brush. As a result, the residual image of the developed pattern is prevented from appearing afterwards. Such a procedure is repeated thereafter.
Some different approaches have heretofore been proposed to charge the developer to a desired polarity sufficiently. For example, the developing roller or similar member for depositing a charge on the developer by friction may contain a polyvinyl alcohol resin on the surface thereof in order to charge the developer to a predetermined polarity sufficiently, as taught in Japanese Patent Laid-Open Publication (Kokai) No. 56-159674 by way of example. Alternatively, the surface of the developing roller may be implemented by a copolymer of vinylester monomers, as proposed in, for example, Japanese Patent Laid-Open Publication No. 56-91262. Further, the surface of the developing roller may be treated with a stylene copolymer, acryl copolymer, polycarbonate, polyamide, polyvinyl chloride, polyvinyl acetate or similar thermoplastic resin to form a resin layer, and then the roller may be bodily subjected to heat treatment at a temperature higher than glass transition point, as disclosed in Japanese Patent Laid-Open Publication No. 57-64268 by way of example.
Regarding a developing device using a two component type developer, i.e., a mixture of toner and carrier, some implementations have been proposed to promote the charging of the developer and which miniaturize and simplify the structure of the device. One of them is to cover the surface of the developing roller with a polyester resin or similar insulating material chargeable to a polarity opposite to the polarity of the toner, and use a discharge plate produced by depositing aluminum on a Mylar or similar insulating film by vacuum evaporation. The discharge plate is connected to ground and held in contact with the developing roller to dissipate the charge of the roller. This kind of scheme is taught in, for example, Japanese Patent Laid-Open Publication 1-169472.
The problem with the conventional developing devices is that fine particles of the developer and additives included in the developer deposit on the surface of the developing roller. This is usually referred to as filming and prevents the developer from being sufficiently charged by friction, thereby reducing the amount of charge. Another problem is that when the charge due to the friction of the developer and roller surface is intense, it sometimes occurs that the surface potential of the roller sequentially increases with the elapse of time. Further, it is likely that the roller surface is frictionally charged by the discharge brush. Such occurrences change the developing characteristic, i.e., the potential of the surface of the photoconductive belt where development begins, and so-called gamma characteristic, adversely effecting images. Therefore, it is preferable that the roller surface be restored to the initial state before the next developing cycle begins.
In the devices taught in previously mentioned Laid-Open Publication Nos. 56-159674 and 56-91262, the surface of the developing roller is positively used to charge the developer. As a result, the potential of the roller surface becomes offset to one polarity with the elapse of time. Hence, some discharging means is required. However, since the roller surface is insulating, the discharging means is apt to charge the roller surface by friction in contact therewith, depending on the material thereof.
The device disclosed in Laid-Open Publication No. 57-64268 has a drawback in that the roller surface is susceptible to moisture since it is implemented by a polar high molecule. In addition, the fine particles of the developer electrostatically adhere to the roller surface fast. As a result, the charging of the developer is obstructed.
Further, the device proposed in Laid-Open Publication No. 1-169472 has, in addition to the problems of Laid-Open Publication No. 57-64268, a problem in that the material and configuration of the discharge plate are likely to prevent the roller surface from being sufficiently charged. Specifically, in a construction taught in No. 1-169472 specifically, aluminum is deposited on the discharge plate by vacuum evaporation. However, aluminum is not feasible for discharging since an insulating layer of aluminum oxide is formed on the surface of aluminum. Moreover, since the discharge plate contacts the roller surface tangentially at a flat portion thereof, it cannot discharge the roller surface sufficiently when, for example, the fine particles of the developer electrostatically adhere to the roller surface fast, as stated earlier.